arm/km: convert mgcoge3un target to km_kirkwood

Use the generic header km_kirkwood.h and get rid of the
board specific header.

changes for v2: rebased because of changes in other patches

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Gerlando Falauto <gerlando.falauto@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>
diff --git a/boards.cfg b/boards.cfg
index 808f85b..003c2af 100644
--- a/boards.cfg
+++ b/boards.cfg
@@ -143,7 +143,7 @@
 km_kirkwood                  arm         arm926ejs   km_arm              keymile        kirkwood    km_kirkwood:KM_KIRKWOOD,KM_DISABLE_PCI
 km_kirkwood_pci              arm         arm926ejs   km_arm              keymile        kirkwood    km_kirkwood:KM_KIRKWOOD_PCI,KM_RECONFIG_XLX
 kmnusa                       arm         arm926ejs   km_arm              keymile        kirkwood    km_kirkwood:KM_NUSA
-mgcoge3un                    arm         arm926ejs   km_arm              keymile        kirkwood
+mgcoge3un                    arm         arm926ejs   km_arm              keymile        kirkwood    km_kirkwood:KM_MGCOGE3UN
 kmcoge5un                    arm         arm926ejs   km_arm              keymile        kirkwood    km_kirkwood:KM_COGE5UN
 portl2                       arm         arm926ejs   km_arm              keymile        kirkwood
 inetspace_v2                 arm         arm926ejs   netspace_v2         LaCie          kirkwood	lacie_kw:INETSPACE_V2